What is an Emergency Shutdown System (ESD) and how is it designed?

Emergency Shutdown System rapidly brings process to a safe state during emergency. Components: sensors (high pressure, toxic gas, fire detection), logic solver (safety PLC or relay-based), and final elements (shutdown valves, trip signals). Design basis: define ESD levels (unit, plant, area), identify critical equipment requiring trip, determine required response time, and specify failure position (fail-safe). Logic: typically hardwired for fastest response, use voting (1oo2, 2oo3) for reliability vs. spurious trip balance. Key principles: independent from process control system, de-energize to trip (fail-safe), bypass management procedures, and regular testing. Standards: IEC 61511 for process safety systems. Documentation includes cause and effect matrix and ESD logic diagrams.